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Annual Review of Astronomy and Astrophysics - Early Publication
Reviews in Advance appear online ahead of the full published volume. View expected publication dates for upcoming volumes.
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An Observational View of Structure in Protostellar Systems
First published online: 09 May 2024More LessThe envelopes and disks that surround protostars reflect the initial conditions of star and planet formation and govern the assembly of stellar masses. Characterizing these structures requires observations that span the near-IR to centimeter wavelengths. Consequently, the past two decades have seen progress driven by numerous advances in observational facilities across this spectrum, including the Spitzer Space Telescope, Herschel Space Observatory, the Atacama Large Millimeter/submillimeter Array, and a host of other ground-based interferometers and single-dish radio telescopes.
- ▪ Nearly all protostars have well-formed circumstellar disks that are likely to be rotationally supported; the ability to detect a disk around a protostar is more a question of spatial resolution rather than whether or not a disk is present.
- ▪ The disks around protostars have inherently higher millimeter/submillimeter luminosities as compared to disks around more-evolved pre-main-sequence stars, though there may be systematic variations between star-forming regions.
- ▪ The envelopes around protostars are inherently asymmetric, and streamers emphasize that mass flow through the envelopes to the disks may not be homogeneous.
- ▪ The current mass distribution of protostars may be impacted by selection bias given that it is skewed toward solar-mass protostars, which is inconsistent with the stellar initial mass function.
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The Physical Origin of the Stellar Initial Mass Function
P. Hennebelle, and M.Y. GrudićFirst published online: 23 April 2024More LessStars are among the most fundamental structures of our Universe. They comprise most of the baryonic and luminous mass of galaxies; synthesize heavy elements; and inject mass, momentum, and energy into the interstellar medium. They are also home to the planets. Because stellar properties are primarily decided by their mass, the so-called stellar initial mass function (IMF) is critical to the structuring of our Universe. We review the various physical processes and theories that have been put forward as well as the numerical simulations that have been carried out to explain the origin of the stellar IMF. Key messages from this review include the following:
- ▪ Gravity and turbulence most likely determine the power-law, high-mass part of the IMF.
- ▪ Depending of the Mach number and the density distribution, several regimes are possible, including ΓIMF ≃ 0, −0.8, −1, or −1.3, where dN/d log M ∝ MΓIMF. These regimes are likely universal; however, the transition between these regimes is not.
- ▪ Protostellar jets can play a regulating influence on the IMF by injecting momentum into collapsing clumps and unbinding gas.
- ▪ The peak of the IMF may be a consequence of dust opacity and molecular hydrogen physics at the origin of the first hydrostatic core. This depends weakly on large-scale environmental conditions such as radiation, magnetic field, turbulence, or metallicity. This likely constitutes one reason for the relative universality of the IMF.
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The Interstellar Medium in Dwarf Irregular Galaxies
First published online: 19 April 2024More LessDwarf irregular (dIrrs) galaxies are among the most common type of galaxy in the Universe. They typically have gas-rich, low surface-brightness, metal-poor, and relatively thick disks. Here, we summarize the current state of our knowledge of the interstellar medium (ISM), including atomic, molecular, and ionized gas, along with their dust properties and metals. We also discuss star-formation feedback, gas accretion, and mergers with other dwarfs that connect the ISM to the circumgalactic and intergalactic media. We highlight one of the most persistent mysteries: the nature of pervasive gas that is yet undetected as either molecular or cold hydrogen, the “dark gas.” Some highlights include the following:
- ▪ Significant quantities of Hi are in far-outer gas disks.
- ▪ Cold Hi in dIrrs would be molecular in the Milky Way, making the chemical properties of star-forming clouds significantly different.
- ▪ Stellar feedback has a much larger impact in dIrrs than in spiral galaxies.
- ▪ The escape fraction of ionizing photons is significant, making dIrrs a plausible source for reionization in the early Universe.
- ▪ Observations suggest a significantly higher abundance of hydrogen (H2 or cold Hi) associated with CO in star-forming regions than that traced by the CO alone.
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The Evolution of Massive Binary Stars
First published online: 19 April 2024More LessMassive stars play a major role in the evolution of their host galaxies and serve as important probes of the distant Universe. It has been established that the majority of massive stars reside in close binaries and interact with their companion stars during their lifetimes. Such interactions drastically alter their life cycles and complicate our understanding of their evolution, but are also responsible for the production of interesting and exotic interaction products.
- ▪ Extensive observation campaigns with well-understood detection sensitivities have enabled the conversion of observed properties into intrinsic characteristics, facilitating a direct comparison to theory.
- ▪ Studies of large samples of massive stars in our Galaxy and the Magellanic Clouds have unveiled new types of interaction products, providing critical constraints on the mass transfer phase and the formation of compact objects.
- ▪ The direct detection of gravitational waves has revolutionized the study of stellar mass compact objects, providing a new window to study massive star evolution. Their formation processes are, however, still unclear. The known sample of compact object mergers will increase by orders of magnitude in the coming decade, which is vastly outgrowing the number of stellar-mass compact objects detected through electromagnetic radiation.
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Dust Growth and Evolution in Protoplanetary Disks
First published online: 19 April 2024More LessOver the past decade, advancement of observational capabilities, specifically the Atacama Large Millimeter/submillimeter Array (ALMA) and Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instruments, alongside theoretical innovations like pebble accretion, have reshaped our understanding of planet formation and the physics of protoplanetary disks. Despite this progress, mysteries persist along the winded path of micrometer-sized dust, from the interstellar medium, through transport and growth in the protoplanetary disk, to becoming gravitationally bound bodies. This review outlines our current knowledge of dust evolution in circumstellar disks, yielding the following insights:
- ▪ Theoretical and laboratory studies have accurately predicted the growth of dust particles to sizes that are susceptible to accumulation through transport processes like radial drift and settling.
- ▪ Critical uncertainties in that process remain the level of turbulence, the threshold collision velocities at which dust growth stalls, and the evolution of dust porosity.
- ▪ Symmetric and asymmetric substructure are widespread. Dust traps appear to be solving several long-standing issues in planet formation models, and they are observationally consistent with being sites of active planetesimal formation.
- ▪ In some instances, planets have been identified as the causes behind substructures. This underlines the need to study earlier stages of disks to understand how planets can form so rapidly.
In the future, better probes of the physical conditions in optically thick regions, including densities, turbulence strength, kinematics, and particle properties will be essential for unraveling the physical processes at play.
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Plurality of Worlds
First published online: 19 April 2024More LessHuman interest in the possibility of other worlds in the Universe has existed for over two millennia. In recent centuries, this question has been translated into the following terms: Are there planetary systems linked to stars other than the Sun?
Developments in astronomical instrumentation have transformed this philosophical dream into a new, vibrant chapter in astronomy. This article describes my journey that started over 40 years ago with the exploration of the dynamics of our Galaxy, that brought astonishing scientific progress to which my collaborators and I have contributed, and eventually led to the amazing discovery of the plurality of worlds.
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